Palaeontologia Electronica
palaeo-electronica.org
Challenges encountered during acid resin transfer preparation of
fossil fish from Monte Bolca, Italy
Mark Graham and Lu Allington-Jones
ABSTRACT
Five specimens of fossil fish from the Eocene deposits of Monte Bolca, Italy, were
selected for preparation by the resin transfer method. The specimens were set in a
synthetic resin, leaving the base exposed, and acetic acid was utilised to remove calcareous matrix from the exposed side. The specimens, of diverse genera, were
obtained from various collectors from 1847 to 1933 and were therefore of both historical and scientific interest. The matrix blocks containing each fish were known to have
been originally adhered onto supporting slabs of the same matrix with an unknown
resin, and the joins concealed with a mortar. This paper records successful outcomes,
and documents various challenges encountered during the preparation process,
together with an account of the materials and techniques utilised. Due to the poor performance of the embedding resin initially chosen, a brief overview of resin types and
trials of some of the commercially available alternatives is included.
Mark Graham, The Conservation Centre, The Natural History Museum, London, SW7 5BD, United
Kingdom, [email protected]
Lu Allington-Jones, The Conservation Centre, The Natural History Museum, London, SW7 5BD, United
Kingdom, [email protected]
Keywords: Acetic; acid; fossil; preparation; resin; deterioration
INTRODUCTION
The Monte Bolca outcrops near Verona, Italy,
are limestone Lagerstätten containing extremely
well-preserved fossil fish of Eocene age. The beds
are thought to represent the earliest record of a
perciform-dominated coral reef (Bellwood, 1996).
The area was discovered in the sixteenth century
and has so far produced around 250 fish species
along with crocodiles, snakes, invertebrates and
plants. Due to their undisturbed but two-dimensional nature, the fossil fish are ideal for prepara-
tion by the resin transfer technique. The transfer of
fossils was first documented at the end of the nineteenth century (Young, 1877; Holm 1890), and the
use of acid was first documented by Bather (1908).
Acids became widely used from the late 1930s
and, following collaboration with zoological colleagues who had been embedding anatomical
specimens in resins, Toombs and Rixon (1950)
were the first to set specimens into a clear resin, to
enable viewing from both sides. Whybrow (1985)
and Lindsay (1986, 1995) provide reviews of the
PE Article Number: 18.2.4T
Copyright: Palaeontological Association May 2015
Submission: 16 February 2015. Acceptance: 27 April 2015
Graham, Mark and Allington-Jones, Lu 2015. Challenges encountered during acid resin transfer preparation of fossil fish from Monte
Bolca, Italy. Palaeontologia Electronica 18.2.4T: 1-9
palaeo-electronica.org/content/2015/1190-challenges-monte-bolca-fish
GRAHAM & ALLINGTON-JONES: CHALLENGES MONTE BOLCA FISH
history and application of acid preparation, whilst
Rutzky et al. (1994) provide a detailed guide to the
various techniques.
The collections at the Natural History
Museum, London, UK (NHM), contain several
specimens prepared by the resin transfer technique. These have been created over several
decades and with varying success. This article outlines the issues encountered during the application
of this technique on five fish from Monte Bolca,
requested for research. It also includes a comparison of a selection of resins, which are currently
commercially available.
THE SPECIMENS
The blocks containing the specimens had
been split laterally and the fossil fish were thus
exposed in a single plane. The specimens prepared were: NHMUK (The Natural History
Museum, United Kingdom) PV OR19011 Ductor
leptosomus (purchased 1844); NHMUK PV
OR21389 Sphyraena bolcensis (purchased 1847);
NHMUK PV P1990 Vomeropsis longispinus (purchased 1882); NHMUK PV P9449 Seriola prisca
(purchased, date unknown); NHMUK PV P16128
Lates gracilis (purchased 1931). In the case of
specimen NHMUK PV P9449 both part and counterpart were held in the collection. The left lateral
side was selected for preparation because more of
the skull was contained within the matrix and thus
offered greater morphological potential from exposure by chemical development.
The blocks all possessed cracks or had been
reconstructed from the original broken sections of
the same specimen, and had been embedded with
unknown mortar and adhesive. Some areas had
also suffered delamination and had been treated
with a clear coating.
FTIR (FOURIER-TRANSFORM INFRA-RED)
SPECTROGRAPHIC ANALYSIS OF THE
HISTORIC RESINS
Resins and adhesives from the fish specimens were analysed using FTIR, as part of the
condition reporting procedure. Samples of the
adhesive from specimens NHMUK PV P1990 and
NHMUK PV OR21389, and a mortar sample from
NHMUK PV P16128 were crushed and incorporated into potassium bromide tablets. These were
analysed using a Perkin Elmer Spectrum One FTIR
Spectrometer and Spectrum software version
5.0.1. Spectra were compiled by averaging 32
scans and normalised against a potassium bro2
mide background. The mortar displayed a profile
between 3500 and 1500 cm-1 which is characteristic of phenolic resin (Koenig, 1999). This is supported by the C-O ester peak at 1081 cm-1. This
phenolic resin, likely to be Bakelite (Ventikou,
1999), was bulked out with calcium carbonate (as
seen in the 1427, 874 and 712 cm-1 peaks), which
is likely to be ground from the bulk matrix surrounding the fossil. The adhesives were both found to be
polyvinyl alcohol (Koenig, 1999), again bulked with
calcium carbonate. The extra peak at 2514 cm-1 in
P1990 is due to the presence of magnesium,
implying a dolomite matrix source. The spectra can
be seen in Figure 1.
ACID TRANSFER TREATMENT
Materials
Synthetic resins are generally used in two to
four different ways during acid transfer: the embedding medium, consolidant for exposed bones and
also as barriers to acid erosion. The latter includes
gap-fills (such as Synocryl 9122x mixed with glass
microballoons) that prevent uncontrolled under-cutting. Maisey et al. (1991) used Glyptal (of unspecified formula) as a consolidant and barrier
combined. However, this was considered unsuitable for the Monte Bolca specimens, since a rigid
and more readily reversible coating was required.
Rixon (1976) found poly (butyl methacrylate) to be
far superior to “Glyptal” and also to poly (methyl
methacrylate) for consolidation. Furthermore,
Rutzky et al. (1994) found that Glyptal Clear 1276
discolours and deteriorates over time, after 10
years specimens required conservation treatment.
Rutzky et al. (1994) recommend Paraloid B67 (a
polyisobutyl methacrylate) and Paraloid B72 (an
ethyl methacrylate-methyl acrylate copolymer) for
use in formic acid, but trials at the NHM have found
that, in acetic acid, this discolours and loses adhesion. Lindsay (1986) documents the use of cellulose nitrate, Alvar 1570 in amyl acetate, Butvar
(poly (vinyl butryal)), Vinalak 5911 and polystyrene
coatings, but warns that most of these are too brittle, too sticky or shrink on curing. He recommends
Synocryl 9123s, a poly (butyl methacrylate). This
was used successfully at the NHM until 2003, but
its production ceased and trials of alternative resins were undertaken. During these trials Synocryl
9122x was found to be the most suitable barrier
coating for fossils during acetic acid immersion
(Schiele, 2008). This is a poly (butyl methacrylate)
thermoplastic acrylic polymer in xylene manufactured by Cray Valley, formerly known as Bedacryl
PALAEO-ELECTRONICA.ORG
FIGURE 1. Fourier-Transform Infrared spectrograph of
the three existing polymers. Wavenumber refers to the
wavelength of light and % Transmission refers to the
amount of each wavelength that is transmitted through
the sample. These are the parameters measured by the
spectrometer to achieve an infra-red spectrum. Each
peak is indicative of a different chemical bond and different substances will produce distinct patterns.
122x. It is heat resistant to 180oC and resistant to
acetic acid, but easily removed with acetone. After
several immersions the edges of the Synocryl
9122x may begin to lift, whilst the matrix is
removed, so in-painting with a fresh layer may be
necessary to prevent undercuts. Microcrystalline
wax has also been used as an effective barrier by
the authors, but it is more suitable for use on areas
of matrix rather than the specimen itself, since the
high temperatures required to apply and remove it
can be problematic.
Rutzky et al. (1994) describe several resin
transfer techniques in great detail, they specify the
use of Alplex Clear Casting Resin 132 (polyester
resin in a styrene monomer) for embedding but
with formic, not acetic, acid. Maisey et al. (1991)
recommend Alpex (a cyclised rubber phenolic
resin, not to be confused with Alplex Clear Casting
132) but find that etching of the cured resin occurs
after prolonged exposure to acid. They suggest the
incorporation of a transparent acrylic window to
counteract this. Evander (1991) used an unspecified polyester resin whilst Rixon (1976) recommends polyester resins in general for embedding
media and mentions Trylon EM301 in particular.
This is no longer available, but Trylon offer two
clear casting polyester resins EM310PA and
EM400PA.
In the 1980s tests were undertaken at the
NHM to discover the most suitable embedding
medium. Epoxy resins, polyester resins and even a
transparent silicon rubber were trialled. Scott
Bader brand Strand Resin C was found to be the
only viable option (Cornish, personal commun.,
2014). Since then, Strand Resin C (polyester resin)
has been used to embed transfer specimens at the
NHM, but this resin is no longer commercially available and an alternative had to be identified. In
2013, a two-part epoxy resin, Araldite 2020, was
chosen because of its long-term chemical stability.
This is an adhesive and laminating resin, which
had been used successfully to manufacture clear
mounts at the NHM and (according to the manufacturers’ website) is also acid resistant. During acid
immersion, however, this resin exhibited undesirable characteristics: a “tackiness” when set, which
inclines it to mark easily; inconsistency of clarity
(some areas remained transparent when set, some
became opaque after acid immersion); overall yellowing after acid immersion; cracking/splintering;
and warping during drying. The acid transfer treatment undertaken using Araldite 2020 is outlined
below.
Processes and Techniques Employed
Acid preparation is considered a destructive
process in accordance with the NHM policies and
procedures. Permission for “destructive sampling”
was therefore first obtained by the researchers
from the museum authorities. Following NHM Conservation Centre best practice guidelines the specimens were then photographed and condition
reports created for each one. A project review,
including the impact on the specimens was also
produced. Labels were removed using a poultice of
methyl cellulose and acid free tissue, left for one
hour and gently peeled away. The labels were then
dried between sheets of silicone release paper,
blotting paper and weighted glass plates, before
storage in inert polyester sleeves. The cement/
mortar was mechanically removed from the edges
of each block using a NSK Electer Emax, model
NE129 rotary cutter fitted with an Edenta 2.2 cm
diameter diamond cutting disk, operated at 15000
rpm. The blocks were partially immersed in acetone baths to loosen the resin between the top and
bottom layers. On specimens NHMUK PV
OR21389, NHMUK PV P1990 and NHMUK PV
P19011 the two layers were pried apart using a
small pallet knife but on specimens NHMUK PV
P9449 and NHMUK PV P16128 the bottom layer
was removed by cutting squares with the rotary
blade into the matrix and carefully chipping out
each cube with a small cold-chisel. Cracks within
each block were filled with an acid-resistant paste
(Synocryl 9122x mixed with glass microballoons) to
avoid uncontrolled acid erosion. The surfaces of
3
GRAHAM & ALLINGTON-JONES: CHALLENGES MONTE BOLCA FISH
FIGURE 2. Specimen NHMUK PV P1990 during preparation. 1, the block split into halves, showing mortar. 2, The
underside of the block set in resin, the majority of the matrix has been removed mechanically. 3, The top of the block,
with the exposed fossil, embedded in resin.
the blocks were then cleaned using a Comco Accuflo micro abrasive blaster and 53 micron aluminium
oxide powder, delivered at 40 psi via a 0.76 mm
diameter nozzle, and the edges of the fossils were
defined using a pin vice so that the surface of the
exposed bones would become as deeply inset into
the resin as possible. Containers were manufactured from flat sided (corrugated interior) polypropylene board to hold the specimens and create the
resin blocks, the two-part resin (Araldite 2020) was
mixed, and a thin layer painted onto the fossil surface with a pure sable artists’ brush to ensure that
no bubbles were formed by trapped air. The
remainder of the resin was then gently poured into
the container and allowed to set for 48 hours. An
acid immersion regime (described below) followed,
consisting of acid immersion, removal of dissolved
matrix with a soft brush and other mechanical
means, rinsing, drying and the protective coating of
exposed bone using Synocryl 9122x.
Figure 2 shows specimen NHMUK PV P1990
during the early stages of this process. Specimen
NHMUK PV P16128 was contained within a darker
matrix, possibly from a near-shore deposit, and
contained a high clay content making it much less
soluble in acid than the other blocks. Following
poor results from acid immersion, this specimen
was almost entirely mechanically prepared within
the cured resin block.
Record of Immersion, Rinsing and Drying
Durations
A mixture of 5% acetic acid and previously
used (spent) acetic acid at 80% water/20% preused solution was used for immersion, further buffered with 5 grams calcium orthophosphate per litre
(Jeppsson et al., 1985; Jeppsson and Anehus,
1995). The pH level was maintained at 3.5 during
immersions. The pre-used solution, referred to by
Jeppsson et al. (1985) as “acetate soup”, contains
4
a concentration of calcium ions (Ca2+) and reduces
the destructive potential of acetic acid on the calcium phosphate fossils. The specimens underwent
eight immersions of 24 hours, each followed by
three days rinsing in gently running tap water (the
jet directed away from the specimen, to prevent
disturbing the bones) and a day air-drying at 19oC
(drying in an oven at higher temperatures was
found to cause warping of the cured resin). The
mechanical removal of non-carbonate residues
from the bones was undertaken prior to drying.
Additional resin was applied to freshly exposed
bone following each drying and allowed to harden
prior to photography and re-immersion in acid.
With the exception of NHMUK PV P16128 relatively little mechanical preparation in between
acid immersions was necessary to the specimens
and consisted of physical removal of residues from
the exposed bones. Nevertheless, due to the very
fragile nature of the small bones, this process was
time-consuming and undertaken under the microscope. In addition, the original resin, which had
been used to adhere the blocks and gap fill,
required removal when it became exposed by the
acid as it would otherwise protrude above the fossil-bearing surfaces. In the case of NHMUK PV
P16128, careful removal of weakened matrix
flakes, which were not fully dissolved by acid, was
carried out with a scalpel and pin under the microscope following each immersion.
Following the eight full-block immersions,
which removed most of the matrix from the specimens, smaller areas of undissolved matrix (mainly
in the cranial areas) were removed by partialimmersion of the specimens in acid to minimise
acid exposure on the rest of the skeletons.
To compensate for the etching and opacity of
the Araldite 2020 following acid immersion, the
cured resin surfaces were rubbed down with wet
and dry fine abrasive paper, rinsed in water and
PALAEO-ELECTRONICA.ORG
FIGURE 3. 1, Yellowed resin shows dimpled (etched)
surface. A glass plate has been added to enhance
clarity. 2, Yellowed resin displaying severe cracking
and near opacity.
thoroughly dried and a second layer of the resin
applied. This initially brought the surfaces to a clear
finish, but these deteriorated again following subsequent immersions. Cracked and broken areas of
Araldite 2020 surrounding specimens NHMUK PV
P1990 and NHMUK OR21389 were removed and
resin re-applied, but each fresh application subsequently failed. Fortunately, the areas affected were
at a sufficient distance from the specimens that no
damage occurred. The project was completed over
a period of 20 weeks.
Determining the Most Suitable Embedding
Resin
Following the poor performance of Araldite
2020, alternative resins were investigated. Traditionally “Strand Resin C” manufactured by Scott
Bader Company (an isopthalic polyester) has been
used at the NHM but this is no longer commercially
available. It also suffers from yellowing, clouding
and will crack if applied in an inappropriately thick
layer (Figure 3). A replacement must perform as
well, or better than Strand Resin C.
The choice of embedding media can be bewildering, because there are many commercially
available synthetic resins. These can be grouped
into types: phenolic, polyester, acrylic and epoxy
resins. Phenolic resin is formed by the reaction of
phenol and formaldehyde (Fibre Max Composites,
2013). It generally requires high curing temperatures and pressures so is not suitable for fossil
transfer. The condensation reaction also produces
volatile by-products (Cripps, 2014a).
There are two main types of polyester resin:
orthophthalic and isophthalic, the latter is more
expensive but generally possesses higher chemical resistance. Most polyester resins are a solution
of polyester in a monomer such as styrene, which
reduces viscosity and enables curing through
cross-linking of the molecular chains of the polyester. A peroxide initiator and an accelerator catalyst
are used to activate vinyl polymerisation, but con-
siderable heat production and shrinkage can occur
during this process (Horie, 1987; Cripps, 2014b).
Vinylester resin is a bisphenol chlorinated polyester resin. It has higher mechanical strength and
resistance to water than polyester resin but
requires high curing temperatures (Fibre Max
Composites, 2013) so would be unsuitable for
embedding fossils.
Acrylic resins are thermoplastics derived from
the polymersiation of acrylic or methacrylic acid.
They have good resistance to oxidation and yellowing with age (Davison and Newton, 2003).
Paraloid B72 is an acrylic resin, an ethyl methacrylate-methacrylate copolymer commonly used in
conservation (Koob, 1986) and preparation (Davidson and Brown, 2012), but it has a relatively low
glass transition temperature of 40oC and has been
found to perform poorly in acetic acid immersion
and drying cycles at the NHM.
Epoxy resins are cured by the addition of a
hardener (usually an aliphatic amine or amide)
rather than a catalyst (Horie, 1987). The hardener
itself reacts with the resin and must therefore be
mixed correctly (Cripps, 2014a). Epoxy resins are
commonly prepared from a reaction of bis-phenol
A and epichlorohydrin (Fibre Max Composites,
2013). They are generally more resistant to degradation than other resin types, possess good water
resistance and low shrinkage during cure (Horie,
1987; Cripps, 2014a).
To assess currently available products and
their suitability for use in acid preparation, a range
of easy to use synthetic resins were investigated.
Six resins (containing no fossil material) were subjected to five cycles of the same immersion regime
as the fish specimens (24 hour immersions in 5%
acetic acid, buffered with calcium orthophosphate
followed by three days of rinsing and then air drying). The resins were Isopthalic Polyester Resin
and Special Water Clear Casting Resin (Synolite
0328-A-1 an orthopthalic resin), both by Fibreglass
Direct; Clear Casting Resin (also an orthopthalic
resin) by Tiranti, with and without 1% UV stabiliser
additive; Araldite 2020 by Huntsman and Epotek301 by Epoxy Technology (both epoxies). These
resins were chosen to represent a range of the
types available on the market. Even though
Araldite 2020 had performed poorly in real situations, it was included in the trials to provide a
benchmark. The Isopthalic resin was tested at both
2% catalyst and at 1%, the latter recommended by
the manufacturer to reduce exothermal effect and
produce a lighter colour. Within an extraction hood,
each resin was mixed according to manufacturer’s
5
GRAHAM & ALLINGTON-JONES: CHALLENGES MONTE BOLCA FISH
TABLE 1. Results of resin trials.
Excessive heat produced
during curing
Colour when
cured
Epo-tek301, 5 mm
No
Colourless
Became soft and yellowed slightly, edges
flaked away
Epo-tek301, 10 mm
No
Colourless
Became soft and yellowed slightly, edges
flaked away
Tiranti Clear Casting, 5 mm
No
Colourless
Colourless, white bloom on surface.
Tiranti Clear Casting, 10 mm
Yes
Colourless
Colourless, white bloom on surface.
Tiranti Clear Casting + UV stabiliser,
5 mm
Yes
Colourless
Colourless, white bloom on surface.
Tiranti Clear Casting + UV stabiliser,
10 mm
Yes
Colourless
Colourless, white bloom on surface.
Isopthalic Resin PA plus 1%
catalyst, 5 mm
Yes
Amber
Amber, severe white bloom on surface.
Isopthalic Resin PA plus 1%
catalyst, 5 mm
Yes
Amber
Amber, severe white bloom on edges.
Isopthalic Resin PA plus 2%
catalyst, 10 mm
Yes
Dark Amber
Amber, slight white bloom on surface.
Isopthalic Resin PA plus 2%
catalyst, 10 mm
Yes
Dark Amber
Amber, severe white bloom on surface.
Araldite 2020, 5 mm
No
Colourless
Moderate yellowing and severe
decomposition of form due to extreme
softening. Remained flexible after
treatment.
Araldite 2020, 10 mm
No
Colourless
Moderate yellowing and severe
decomposition of form due to extreme
softening. Edges and flaked pieces are
flexible.
Resin and layer thickness
Water White Clear Casting, 5 mm
No
Colourless
Clear, colourless.
Water White Clear Casting, 10 mm
No
Colourless
Colourless, slightly frosted in places.
instructions and then poured into two shallow trays
to set (personal protective equipment was worn
according to the Material Safety Data Sheets for
each set of chemicals). 10 ml was poured into one
tray, to produce a layer approximately 5 mm in
thickness, whilst 40 ml was poured into the second
tray to produce a layer just over 10 mm. The difference in thickness was tested because thick layers
are more desirable for stability during treatment,
but can undergo shrinkage cracking and produce
higher temperatures during curing (Davison and
Newton, 2003).
The ideal properties of an embedding resin
would be long-term chemical stability, resistance to
immersion in acetic acid, low volatile emission
during curing, unreactive with the specimen or
matrix, excellent optical clarity, low temperature
generation during curing, and no shrinkage, cracking or warping during curing or throughout immersion cycles.
Table 1 shows the results of the resin trials.
The Water White Clear Casting (Synolite 0328-A6
Appearance following acid treatments
1) gave the best results for clarity, low heat generation, rigidity and structural integrity. Although the
surface on some samples developed a slight frosting effect, this can be remedied by painting on a
layer of the same resin after all cycles have been
completed.
CONCLUSIONS
Insofar as historic specimens of fossil fish
from the Monte Bolca location are concerned,
where the fossil-bearing slab has been attached to
a second, supporting slab of matrix, the preparator
should anticipate a significant amount of mechanical preparation to be necessary (in order to reduce
block depth) to augment the (traditionally) chemical-only acid-resin transfer technique. When
removing old resin, such as the dark brown Bakelite encountered during this project, immersion in
solvents should be avoided because the resultant
contaminated fluid can stain the specimen and be
difficult to remove. Furthermore, cured resins,
PALAEO-ELECTRONICA.ORG
FIGURE 4. NHMUK PV P1990 after complete preparation.
when softened, can form a coating on rotary
blades, effectively blunting the abrasive surfaces
and cutting edges. Therefore careful prising apart
of the blocks, where practicable, or cutting matrix
into cubes, which can be levered off with a blunt
blade is recommended.
The exposed bones were effectively protected
during acid immersion by the addition of spent acetic acid and Calcium orthophosphate, which elevate the Ca2+ ion concentration in the solution.
Following rinsing and drying, the bones were successfully protected by applying (with a small, soft
brush) the acid-resistant resin Synocryl 9122x in
50% solution with acetone. Figure 4 shows the
results upon completion of acid preparation of
specimen NHMUK PV P1990. Araldite 2020 was,
however, found to be unsuitable as an embedding
resin and required several stages of restoration.
The initial failures and tests demonstrate that
new resins must be trialled before application,
regardless of manufacturer’s claims, and that resins of similar types perform very differently. Not all
polyesters, for example, cure at low enough temperatures or resist discolouration. Following these
trials, Synolite 0328-A-1 was subsequently tested
on real specimens and performed extremely well
(Figure 5). Based upon the experience with these
specimens, and the resin tests documented herein,
the authors recommend that Synolite (a pre-accelerated, thixotropic, low styrene emission, orthophthalic based unsaturated polyester resin) such as
Water White Clear Casting resin (Synolite 0328-A1) by Fibreglass Direct, be used when the transfer
method of acid preparation is undertaken.
FUTURE PROSPECTS
A new protocol for the resin transfer method
should be introduced. All new resins should be trialled before treatment commences and “blanks” of
resin should be catalogued, dated and associated
with the treatment records for the prepared specimens.
7
GRAHAM & ALLINGTON-JONES: CHALLENGES MONTE BOLCA FISH
FIGURE 5. Oxford University Museum specimen CM4312 Blochius longirostris, set in Synolite 0328-A-1.
ACKNOWLEDGEMENTS
We thank M. Friedman, Oxford University
Earth Sciences and Z. Johanson of the NHM for
consultation during preparation of the specimens.
We also thank C. Collins of the Conservation Centre (NHM) and the editor and reviewers for constructive suggestions for improvement of this
manuscript.
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